1. Field of the Invention
The present invention relates to an apparatus for manufacturing a single crystal rod from a polycrystal feed rod, said apparatus comprising a closed chamber, at which chamber the feed rod is located, said chamber comprising an annular energy supply arranged around the feed rod for melting off the one end of the rod for providing single crystals, said apparatus comprising first moving means for axial movement of the feed rod and second moving means for a rotating relative movement between the feed rod and the annular energy supply.
The present invention also relates to a method of manufacturing a single crystal rod from a polycrystal feed rod, said feed rod comprising a melting zone at the end of the rod, which melting zone is provided by means of an annular energy supply arranged around the feed rod, and in that, in the area below the melting zone single crystals are provided, which feed rod is, by means of first means moved in axial direction, and that, by the second means, a rotating relative movement is provided between the surface of the feed rods and the energy supply.
2. Description of the Related Art
In connection with the manufacture of single crystals, it is known to use the so-called float-zone technique. The technique is used in particular for the manufacture of silicon single crystals.
Application of the float-zone alias floating zone alias float zone technique (FZ) to grow silicon single crystals were first reported by Keck and Golay. The FZ crystals are purer than Czochralsky zone technique (Cz) ingots since the silicon melt used for crystal growth is not contained by a crucible. Large diameter silicon ingots are more difficult to grow by the FZ than by Cz technique. However, the progress of the FZ technique has been able to keep pace with the Cz, and crystals with diameters of between 150 and 200 mm in R&D have been grown in production.
First, the bottom end of a polysilicon rod is preheated. This end of the rod is ground into a v-shape and is placed in the centre of a water cooled, single-turned copper induction coil. A conducting susceptor e.g. graphite is then placed underneath the polysilicon rod with a minimal gap. When an rf current is applied to the copper coil, and electrical eddy current is induced in the susceptor and the temperature of the susceptor increases. The heat is then transferred to the polysilicon rod by radiation. Once the portion of polysilicon in close proximity to the susceptor starts to glow, the eddy current can be induced in this segment of silicon by the rf energy. The graphite susceptor is no longer needed and is removed from the rf coil. The heat is continuously applied until the cone segment of the polysilicon rod melts. Subsequently, a seed is dipped into the molten silicon from below.
Once the seed is wetted by the molten silicon, the growth of a crystal can be initiated by lowering the seed. The polysilicon rod also need to be lowered, but with a much slower rate. As in the Cz technique, dislocation-free growth should be initiated during the seeding process by using fast pull rates. Once the dislocation-free structure is observed (due to the appearance of strong side facets), the ratio of pull rates between the seed and polyrod is gradually decreased so that the crystal diameter will gradually increase. A dopant gas can also be introduced during the growth process.
A critical factor for successful float-zone growth is maintaining the stability of the molten zone. The zone is stable when the inward pressure of the zone is greater than the outward pressure. The inward pressure includes surface tension, cohesion between the solid and liquid, and electromagnetic pressure due to the rf field. The latter two terms are relatively small as compared to the surface tension. The outward pressure mainly includes the hydrostatic pressure resulting from the gravitational force of the molten zone. The hydrostatic pressure is directly proportional to the zone height. Therefore, the molten zone should be kept as narrow as possible.
Frequency of the rf energy is also an important parameter. It is inversely proportional to the penetration depth of the rf energy and electromagnetic force on the melt. It has been suggested that the optimum frequency is between 2 and 3 MHz. When the frequency is below 500 kHz, an undesirable surface melting can occur. On the other hand, a frequency higher than 3 MHz increases the possibilities for arcing.
The FZ technique is used e.g. in WO 01/06041.
However, in connection with the manufacture of single crystals, it is a problem that the feed rod as such that forms the starting point for forming a single crystal rod, essentially must be a circular cylinder with the walls parallel to the longitudinal axis of the cylinder. Likewise it is also required that the surface of the feed rod of a polycrystal rod is smooth and has a low degree of roughness. It is therefore necessary that the feed rod initially to forming the basis of the manufacture of single crystals undergoes a processing, which—in addition to a purification—also includes grinding, whereby the surface is caused to appear fairly smooth, and that the cylinder rod is a circular cylinder, imperfections thus being removed.